PM / devfreq: tegra30: Add error message for devm_devfreq_add_device()

It's difficult to notice that driver failed to probe by looking at KMSG
when devm_devfreq_add_device() fails. Add explicit error message for this
case.

Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>